c++: Improve location information in constant evaluation

c++: Improve location information in constant evaluation

This patch updates 'input_location' during constant evaluation to ensure
that errors in subexpressions that lack location information still
provide accurate diagnostics.

By itself this change causes some small regressions in diagnostic
quality for circumstances where errors used 'input_location' but the
location of the parent subexpression doesn't make sense, so this patch
also includes a small diagnostic improvement to fix the most egregious
case.

gcc/cp/ChangeLog:

	* constexpr.cc (modifying_const_object_error): Find the source
	location of the const object's declaration.
	(cxx_eval_constant_expression): Update input_location to the
	location of the currently evaluated expression, if possible.

libstdc++-v3/ChangeLog:

	* testsuite/25_algorithms/equal/constexpr_neg.cc: Update diagnostic
	locations.
	* testsuite/26_numerics/gcd/105844.cc: Likewise.
	* testsuite/26_numerics/lcm/105844.cc: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-48089.C: Update diagnostic locations.
	* g++.dg/cpp0x/constexpr-70323.C: Likewise.
	* g++.dg/cpp0x/constexpr-70323a.C: Likewise.
	* g++.dg/cpp0x/constexpr-delete2.C: Likewise.
	* g++.dg/cpp0x/constexpr-diag3.C: Likewise.
	* g++.dg/cpp0x/constexpr-ice20.C: Likewise.
	* g++.dg/cpp0x/constexpr-mutable3.C: Likewise.
	* g++.dg/cpp0x/constexpr-recursion.C: Likewise.
	* g++.dg/cpp0x/overflow1.C: Likewise.
	* g++.dg/cpp1y/constexpr-89285.C: Likewise.
	* g++.dg/cpp1y/constexpr-89481.C: Likewise.
	* g++.dg/cpp1y/constexpr-tracking-const14.C: Likewise.
	* g++.dg/cpp1y/constexpr-tracking-const16.C: Likewise.
	* g++.dg/cpp1y/constexpr-tracking-const18.C: Likewise.
	* g++.dg/cpp1y/constexpr-tracking-const19.C: Likewise.
	* g++.dg/cpp1y/constexpr-tracking-const21.C: Likewise.
	* g++.dg/cpp1y/constexpr-tracking-const22.C: Likewise.
	* g++.dg/cpp1y/constexpr-tracking-const3.C: Likewise.
	* g++.dg/cpp1y/constexpr-tracking-const4.C: Likewise.
	* g++.dg/cpp1y/constexpr-tracking-const7.C: Likewise.
	* g++.dg/cpp1y/constexpr-union5.C: Likewise.
	* g++.dg/cpp1y/pr68180.C: Likewise.
	* g++.dg/cpp1z/constexpr-lambda6.C: Likewise.
	* g++.dg/cpp1z/constexpr-lambda8.C: Likewise.
	* g++.dg/cpp2a/bit-cast11.C: Likewise.
	* g++.dg/cpp2a/bit-cast12.C: Likewise.
	* g++.dg/cpp2a/bit-cast14.C: Likewise.
	* g++.dg/cpp2a/constexpr-98122.C: Likewise.
	* g++.dg/cpp2a/constexpr-dynamic17.C: Likewise.
	* g++.dg/cpp2a/constexpr-init1.C: Likewise.
	* g++.dg/cpp2a/constexpr-new12.C: Likewise.
	* g++.dg/cpp2a/constexpr-new3.C: Likewise.
	* g++.dg/cpp2a/constinit10.C: Likewise.
	* g++.dg/cpp2a/is-corresponding-member4.C: Likewise.
	* g++.dg/ext/constexpr-vla2.C: Likewise.
	* g++.dg/ext/constexpr-vla3.C: Likewise.
	* g++.dg/ubsan/pr63956.C: Likewise.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>